That doesn't work so try the next simplest: . That works so it's all you need. Consider it a rule of thumb.

There is a way of getting it formally but I don't have the time to type out the details right now. Chris likes D.E.'s ... he might get a kick out of doing it.

Thanks again,

I handed my work in and it all looks good, however some of my friends said that they did it by using the form .
You said here that that form wouldn't work and to move on to the next form up, so out of interest when I got home I tried it with form: and lo and behold I got the same answer as with form: .